I-Reporters provided some of the earliest images of the devastation to CNN.

Witness Mark LaCroix was the first I-Reporter to send photos to CNN. LaCroix, who watched from his apartment building, described how the devastation unfolded to Wolf Blitzer.

"I heard this massive rumbling, shaking basically," LaCroix said. "I looked out my window and saw the last few seconds of the collapse." Watch his interview »

Ryan Broshar saw the collapse while he was riding his bike on the 10th Avenue bridge, which runs parallel to the Interstate 35 West bridge.

"I just heard a big 'boom' then there was dust everywhere! I pulled my bike over to the side of the road and looked over, and it was unreal," Broshar said. "People getting out of their cars unscathed, and people getting pulled out of their cars unconscious."

He said he called 911 to alert rescuers to the incident, and grabbed photos of the scene with his cell phone camera just seconds after the bridge fell.

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Some of the most incredible images of the tragedy were shot by a recent high school graduate.Andrew Worrall, 19, was driving to the Minnesota Twins baseball game when the bridge gave way.

Worrall arrived on the scene about 15 minutes after the collapse. He used his high school press credentials to get close enough to document the disaster.

"There was a lot of chaos," Worrall told CNN's Kiren Chetry. "Emergency crews were working as hard as they could to get down to the river banks to reach people who were stuck or under water."

Many I-Reporters said they felt compelled to tell the story of what was going on around them. Tim Davis said it was sheer instinct that drove him to take his camera out to the site with his friend, shortly after the collapse occurred.

He ran out of the car and around the wreckage from a crushed train, and then headed down to an embankment. There, he saw cars "crumpled upside down like tin cans" after being mangled and smashed by the collapse of the bridge.

He scrambled down toward the remains of the bridge, grabbing onto rocks and trees on the way down. He broke his sandal, limiting his ability to get around the area.

By the side of the water, he saw a few injured people and tried to comfort them. Before he left, he grabbed a few more pictures of the scene.

A former student at the University of Minnesota, Davis said students travel through the area all the time, and he's become very used to seeing it there. Seeing an empty space was a shock.

"I remember looking and there was nothing there," Davis said. "There was just sky. It was unreal."E-mail to a friend